Heart failure is a debilitating disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the tissues and organs of the body. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract. Often, the ventricles do not adequately fill with blood between heartbeats and the valves regulating blood flow become leaky, allowing regurgitation or back-flow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness and the inability to carry out daily tasks may result. Not all heart failure patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive. As heart failure progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds cardiac muscle causing the ventricles to grow in volume in an attempt to pump more blood with each heartbeat, i.e. to increase the stroke volume. This places a still higher demand on the heart's oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result, typically in the form of myocardial ischemia or myocardial infarction. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output. A particularly severe form of heart failure is congestive heart failure (CHF) wherein the weak pumping of the heart leads to build-up of fluids in the lungs and other organs and tissues.
Heart failure has been classified by the New York Heart Association (NYHA) into four classes of progressively worsening symptoms and diminished exercise capacity. Class I corresponds to no limitation wherein ordinary physical activity does not cause undue fatigue, shortness of breath, or palpitation. Class II corresponds to slight limitation of physical activity wherein such patients are comfortable at rest, but wherein ordinary physical activity results in fatigue, shortness of breath, palpitations or angina. Class III corresponds to a marked limitation of physical activity wherein, although patients are comfortable at rest, even less than ordinary activity will lead to symptoms. Class IV corresponds to inability to carry on any physical activity without discomfort, wherein symptoms of heart failure are present even at rest and increased discomfort is experienced with any physical activity.
The current standard treatment for heart failure is typically centered on drug treatment using angiotensin converting enzyme (ACE) inhibitors, diuretics or digitalis. Cardiac resynchronization therapy (CRT) may also be employed, if a bi-ventricular pacing device is implanted. Briefly, CRT seeks to normalize asynchronous cardiac electrical activation and resultant asynchronous contractions associated with heart failure by delivering synchronized pacing stimulus to both ventricles. The stimulus is synchronized so as to improve overall cardiac function. This may have the additional beneficial effect of reducing the susceptibility to life-threatening tachyarrhythmias. CRT and related therapies are discussed in, for example, U.S. Pat. No. 6,643,546 to Mathis et al., entitled “Multi-Electrode Apparatus And Method For Treatment Of Congestive Heart Failure”; U.S. Pat. No. 6,628,988 to Kramer et al., entitled “Apparatus And Method for Reversal of Myocardial Remodeling With Electrical Stimulation”; and U.S. Pat. No. 6,512,952 to Stahmann et al., entitled “Method And Apparatus For Maintaining Synchronized Pacing”.
In view of the potential severity of heart failure, it is highly desirable to detect its onset within a patient and to track its progression or regression so that appropriate therapy can be provided. Many patients suffering from heart failure already have pacemakers or ICDs implanted therein or are candidates for such devices. Accordingly, it is desirable to provide such devices with the capability to automatically detect and track heart failure and, heretofore, a number of attempts have been made to monitor heart failure using implantable cardiac stimulation devices. For many patients with heart failure, the heart failure results in a generally higher heart rate for a given level of patient activity because of the poor pumping ability of the heart. More specifically, due to reduced stroke volume caused by heart failure, the heart must beat at a faster rate to meet the physiological demands of the patient. By equipping the implanted device with an activity sensor, the relationship between heart rate and activity can thereby be exploited to monitor heart failure.
See, for example, U.S. Pat. No. 6,190,324 to Kieval et al., entitled “Implantable Medical Device for Tracking Patient Cardiac Status”, which describes an implantable medical device for monitoring CHF based on, inter alia, activity levels and heart rate. In one example, the ratio of heart rate to activity level is measured and used to monitor CHF. A potential disadvantage of the technique of Kieval et al., at least insofar as heart rate is concerned, is that the technique appears to detect and utilize the absolute heart rate of the patient. It is believed to be preferable to instead evaluate heart rate relative to the rest rate of the patient or relative to some measure of the heart rate reserve (HRR) of the patient (i.e. a measure of the difference between the rest rate and the maximum heart rate of the patient) so as to obtain a more useful measure of heart rate for the purposes of heart failure evaluation. Also, it appears that many of the exemplary techniques of Kieval et al. are directed to exploiting heart rate values obtained regardless of whether the patient is active or not. The present inventors have recognized that it is preferable to instead evaluate heart failure using heart rate values obtained only while the level of patient activity exceeds some predetermined minimum threshold, so as to permit a more effective evaluation of heart failure. In this regard, Kieval et al. mentions that capture of a heart rate activity coefficient (HRAC) value may be triggered based on either heart rate or activity levels. However, Kieval et al. does not appear to describe techniques that utilize any and all heart rate values obtained while patient activity exceeds a predetermined minimum. Instead, Kieval et al. describes, for example, the use of heart rate values obtained only within a specific range of activity level values.
Accordingly, it is desirable to provide techniques for automatically detecting and tracking heart failure based on patient heart rate and activity levels, which specifically take into account patient rest rate or HRR and which specifically exploit heart rate values measured only while the patient is active.